1. Field of the Invention
The present invention relates to medical implants and, more particularly, to medical implants for the correction of an unstable parts of the spine by joining two or more vertebrae.
2. Description of Related Art
Back pain remains a major public health problem, especially among aged people. Persistent and severe back pain often causes debility and disability, and this pain is closely associated with intervertebral disc abnormalities of the spine.
The human spine is a flexible structure comprised of thirty-three vertebrae. Intervertebral discs separate and cushion adjacent vertebrae, act as shock absorbers, and allow bending between the vertebrae. An intervertebral disc comprises two major components: the nucleus pulposus and the annulus fibrosis. The nucleus pulposus is centrally located in the disc and occupies 25–40% of the disc's total cross-sectional area. The annulus fibrosis surrounds the nucleus pulposus and resist torsional and bending force applied to the disc. Vertebral end-plates separate the disc from the vertebrae on either side of the disc.
As a result of exertion, injury, illness, accident or abuse, one or more of the vertebrae and/or one or more discs may become damaged and malfunctional. Specifically, disorders of the vertebrae and discs include but are not limited to 1) disruption of the disc annulus such as annular fissures; 2) chronic inflammation of the disc; 3) localized disc herniations with contained or escaped extrusions; and 4) relative instability of the vertebrae surrounding the disc.
Various approaches have been developed to treat back pain. Minor back pain can be treated with medication and other non-invasive therapy. However, it is often necessary to remove at least a portion of the damaged and/or malfunctioning back component. For example, when a disc becomes ruptured, a discectomy surgical procedure can be performed to remove the ruptured disc and to fuse the two vertebrae between the removed disc together.
Spinal fusion is indicated to provide stabilization of the spinal column for disorders such as structural deformity, traumatic instability, degenerative instability, and post resection iatrogenic instability. Fusion, or arthrodesis, can thus be achieved, for example, by the formation of an osseous bridge between adjacent motion segments. The fusion can be accomplished either anteriorly between contiguous vertebral bodies or posteriorly between consecutive transverse processes, laminae or other posterior aspects of the vertebrae. Typically, the osseous bridge, or fusion mass, is biologically produced by recreating conditions of skeletal injury along a “fusion site” and allowing the normal bone healing response to occur. This biologic environment at a proposed fusion site requires the presence of osteogenic or osteopotential cells, adequate blood supply, sufficient inflammatory response, and appropriate preparation of local bone. To this end, a process known as decortication is typically used to prepare bone and increase the likelihood of fusion. Decortication involves removing the outer cortex of spinal bone with a burr to induce bleeding bone and release bone marrow. Decortication also initiates the inflammatory response, releases osteoinductive cytokines, provides additional osteogenic cells, and creates a host attachment site for the subsequent fusion mass. Bone graft materials are often used to promote spinal fusions. Autogenous iliac crest cortico-cancellous bone is presently a widely-used bone grafting material.
FIG. 1 illustrates two adjacent vertebrae 21 and 23 of a human spine. The first vertebra 21 comprises left and right transverse processes 25 and 27, respectively, and further comprises a first spinous process 29. The first vertebra 21 further comprises left and right superior articular processes 31 and 33, respectively, and comprises left and right inferior articular processes 35 and 37, respectively. Similarly, the second vertebra 23 comprises left and right transverse processes 45 and 47, respectively, and further comprises a second spinous process 49. The second vertebra 23 further comprises left and right superior articular processes 51 and 53, respectively, and comprises left and right inferior articular processes 55 and 57, respectively. A cross-sectional view of the vertebrae 23, taken along the line 2—2 of FIG. 1, is shown in FIG. 2.
In FIG. 3 an autogenous iliac crest cortico-cancellous bone is grafted onto the two spinous processes 27 and 47 of the two vertebrae 21 and 23. Donor cortical bone from the iliac crest is cut into small rectangular grafts 61 which are placed over the partially-decorticated spinus processes 27 and 47. Donor cancellous bone from the iliac crest is placed between the rectangular grafts 61 and the spinus processes 27 and 47. This autograft process has a shortcoming, however, because of the additional surgery that is required to harvest the autogenous donor materials. The additional surgery can increase the risk of infection and can reduce structural integrity at the donor site. Furthermore, many patients complain of significant pain for several years after the surgery.
In early spinal fusion techniques, bone material, or bone osteogenic fusion devices, were simply disposed between adjacent vertebrae, typically at the posterior aspect of the vertebrae. In the early history of these osteogenic fusion devices, the osteogenic fusion devices were formed of cortical-cancellous bone. Consequently, the spine was stabilized by way of screws, plates and/or rods spanning the affected vertebrae. With this technique, once fusion occurred across and incorporating the bone osteogenic fusion device, the hardware used to maintain the stability of the spine became superfluous.
Following the successes of the early fusion techniques, focus was directed to modifying the device placed within the intervertebral space to support and fuse together adjacent vertebrae by posterior-fusion or anterior grafting. For example, surgical prosthetic implants for vertebrae described in U.S. Pat. No. 5,827,328 include rigid annular plugs that have ridged faces to engage adjacent vertebrae to resist displacement and allow ingrowth of blood capillaries and packing of bone graft. These annular implants are usually made of biocompatible carbon fiber reinforced polymers, or traditional orthopaedic implant materials such as nickel, chromium, cobalt, stainless steel or titanium. The individual implants are internally grooved and are stacked against each other to form a unit between the two adjacent vertebrae.
Another intervertebral fusion device described by Kozak et al. (U.S. Pat. No. 5,397,364) includes an assembly of two lateral spacers and two central spacers, which defines a channel in the center of the fusion device for insertion of bone graft material. The spacers are maintained in their configuration within the intradiscal space by screws threaded into a vertebra from the outside of the disc.
Cylindrical hollow implants or “cages” are represented by the patents to Bagby, U.S. Pat. No. 4,501,269; Brantigan, U.S. Pat. No. 4,878,915; Ray, U.S. Pat. No. 4,961,740; and Michelson, U.S. Pat. No. 5,015,247. The outer wall of the cage creates an interior space within the cylindrical implant that is filled with bone chips, for example, or other bone growth-inducing material. The cylindrical implant can include a threaded exterior to permit threaded insertion into a tapped bore formed in the adjacent vertebrae. One fusion cage implant is disclosed in U.S. Pat. No. 5,026,373 to Ray et al. The Ray '373 fusion cage includes apertures extending through its wall which communicate with an internal cavity of the cage body. The adjacent vertebral bone structures communicate through the apertures with bone growth inducing substances within the internal cavity to unite and eventually form a solid fusion of the adjacent vertebrae. Other prosthetic implants are disclosed in U.S. Pat. Nos. 4,501,269, 4,961,740, 5,015,247 and 5,489,307. Other fusion implants have been designed to be impacted into the intradiscal space.
Experience over the last several years with these interbody fusion devices has demonstrated the efficacy of these implants in yielding a solid fusion. Variations in the design of the implants have accounted for improvements in stabilizing the motion segment while fusion occurs. Nevertheless, some of the interbody fusion devices still have difficulty in achieving a complete fusion, at least without the aid of some additional stabilizing device, such as a rod or plate. Moreover, some of the devices are not structurally strong enough to support the heavy loads and bending moments applied at certain levels of the spine, namely those in the lumbar spine.
Even with devices that do not have these difficulties, other less desirable characteristics exist. Recent studies have suggested that the interbody fusion implant devices, or cages as they are frequently called, lead to stress-shielding of the bone within the cage. It is well known that bone growth is enhanced by stressing or loading the bone material. The stress-shielding phenomenon relieves some or all of the load applied to the material to be fused, which can greatly increase the time for complete bone growth, or disturb the quality and density of the ultimately formed fusion mass. In some instances, stress-shielding can cause the bone chips or fusion mass contained within the fusion cage to resorb or evolve into fibrous tissue rather than into a bony fusion mass.
A further difficulty encountered with many fusion implants is that the material of the implant is not radiolucent. Most fusion cages are formed of metal, such as stainless steel, titanium or porous tantalum. The metal of the cage shows up prominently in any radiograph (x-ray) or CT scan. Since most fusion devices completely surround and contain the bone graft material housed within the cage, the developing fusion mass within the metal cage between the adjacent vertebrae cannot be seen under traditional radiographic visualizing techniques and only with the presence of image scatter with CT scans. Thus, the spinal surgeon does not have a means to determine the progress of the fusion, and in some cases cannot ascertain whether the fusion was complete and successful.
The field of spinal fusion can be benefited by an intervertebral fusion device that beneficially attenuates or eliminates the risk of stress-shielding of the fusion mass, and that also provides for visualization of the fusion mass as the arthrodesis progresses.